Does anyone know exactly how, when and under what conditions the 8 to 4 cylinder trick works?
I know from the "dealer" brochure that both the intake and exhaust valves close in four cylinders acting as "springs", but under what conditions?

 

Check out this thread: MDS "ON" Indicator Light

magnuman has even more detail about it on lxforums.com

 

As I recall, there is electric solinoid that cuts the oil off to four of the hydralic lifters. This bassically shuts the valves down but I can't remember if it was only the intake side or both exhaust and intake. Without the oil pressure in the lifter it can not push the connecting rod which actuates the valve, thus making those four cylinders inactive.

Not sure if the spark is still active but I would think so, wouldn't hurt anything to have the sparkplugs keep firing. I can't tell you how long it takes the hydralic valves to presurize and depresurize. I would think it is within a second since most people think the transition from 4 to 8 cylinders is fairly seemless.

In the not to distant future, the valves will be actuated by electric motors, similar to the fuel injectors. This will allow any the cylinders to be deactivated and infinitly variable valve timing, thus doing away with the camshaft, lifters, push rods, and valve springs. This technology should also eliminate valve float at high speed.

 

Cdn 2 Usa,
It works good. 25.5 MPG, with foot and hills. MDS will not work for more than 6 minutes straight, in order to keep Temperatures consistant. There were some very detailed threads on this subject. Search the previous threads and you should find your answer...